1. Field of the Invention
The invention concerns a plasma torch and a method of fabricating a glass preform to be used in the production of an optical fiber, and in particular a method of glazing the external surface of the preform. It also concerns a preform fabrication system using that method.
2. Description of the Prior Art
As is known in the art, a preform to be used in the production of an optical fiber by a drawing operation is conventionally fabricated or surfaced by depositing silica in successive concentric layers onto a primary preform and by glazing the surface of the outside layer of the resulting cylindrical member.
A preform fabrication method known in the art starts from a cylindrical primary preform onto which silica is deposited in concentric layers by heating the preform locally, using inductive heating means of the plasma torch type, and depositing molten silica obtained from grains of silica injected into the plasma at the mouth of the torch. The molten silica is incorporated into the heated silica at a high temperature in the area of the outside surface of the preform on which the plasma impinges.
Some of the silica melted in this way evaporates because of the high temperatures reached in the plasma and tends to be deposited in the form of soot on cooling. These deposits of soot can be a particular problem because they affect in particular outside areas of the preform which are cooler than the target area. Such deposits are also produced in abundance when one of the two ends of the preform must be heated locally so that it can be cut and the preform obtained in this way worked at one end to have a conical pointed shape which must be of the same optical quality as the rest of the preform for subsequent production of a fiber by hot drawing.
It is known in the art to glaze the outside surface of a preform by heating the preform by means of a plasma torch under conditions corresponding to those for depositing a layer of silica, but without feeding any silica. This improves the surface state of the preform because the plasma vitrifies some of the soot present on the preform. The action of the plasma evaporates a non-negligible quantity of silica because of the intense heating of the target area of the outside surface of the preform. As is known in the art, technological constraints rule out reducing the power of the plasma, which would otherwise be desirable to prevent evaporation of silica during glazing. Soot is therefore created and deposited onto the parts of the preform which are sufficiently cool, and in particular onto the portion of the outside surface of the preform where the glazing operation has just been carried out. Soot remaining on the outside surface of a preform constitutes a serious problem in terms of quality and compromises subsequent drawing operations performed to obtain a fiber.
Attempts have therefore been made to reduce the production of soot by increasing the speed of movement in translation of the preform relative to the plasma torch. The drawback of this solution is that it increases the internal stresses which inevitably occur when the preform is heated by a plasma because of its instantaneous localization to a limited area of the preform and the sudden variations in temperature which then occur between the strongly heated area and the adjoining areas of the preform. It is known in the art to eliminate the internal stresses produced in a preform by annealing in a furnace, but this takes significantly longer than a glazing operation; it conventionally takes about 15 hours.
Document EP-0 887 318 mentions that, to avoid the problems associated with the formation of soot during the operation of drawing a large-diameter preform into a smaller diameter intermediate preform using an oxygen-hydrogen torch, the soot is expelled by a flow of gas with a high flowrate, of the order of 150 l/min.
The object of the present invention is therefore to provide a plasma torch which in particular prevents the deposition of soot during the glazing of an optical fiber preform.